The Optical Transport Hierarchy (OTH) is a transport hierarchy developed on the basis of optical synchronization network SDH/Sonet, and it has the advantage of large cross particle, service transparent transmission and high package efficiency. In modern society, the data service is explosively increased, and the previous optical synchronization network based on circuit switch is not suitable for transmitting such data. Although the data service can be transmitted on the framework of SDH virtual container through the multi-service transport platform, this transport hierarchy has disadvantages such as high mapping cost, low mapping efficiency and complicated technique and it cannot meet the transmission and exchange requirements of a large amount of data service.
The optical transport network re-defined a transport hierarchy which applies a new package format to package data service and the synchronous transport hierarchy (such as SDH/Sonet) service into a uniform structure, thus reducing the mapping hierarchy of the data service and guarantee the transparent transmission of the service. Packaging the optical service makes the original wave division system have the management based on optical multiplexing section layer, and thus networking is more flexible and there is space to upgrade into intelligent optical network. Compared with the present SDH/Sonet transport hierarchy, it has evident advantages and is the direction of development of the optical network.
However, a new transport hierarchy would not appear without foundation and it will not rapidly replace the existing network. It is a process of gradual development. The new transport hierarchy will coexist with the present network, develop in the present network and gradually replace the present network to play a leading role.
The G.709 specification established by ITU-T standards organization defines and regulates the OTN.
FIG. 1 shows a part of OTN mapping defined by the G.709 specification established by ITU-T standards organization, and the mapping path after OTU is not used in the present invention, so it is omitted.
The user signals are in the mapping path of the OTU (Optical Transport Unit) system shown in FIG. 1, and each user signal is input to the OPU (Optical Payload Unit) and then enter into OTU (Optical Transport Unit) through the ODU (Optical Data Unit), wherein the user signals can be SDN signals or other data signals such as STM-16, STM-64, STM-256, and so on.
As shown in FIG. 1, ITU-T G.709 defines three kinds of OPU payload structures and their corresponding speeds. Wherein:
OPU1 payload has the same speed as STM-16 has;
OPU2 payload is added with stuff columns on the basis of STM-64;
OPU3 payload is added with stuff columns on the basis of STM-256.
TABLE 1three kinds of OPU payload structures and their corresponding speedsOPU payload bitOPU typeOPU payload nominal bit raterate toleranceOPU12 488 320kbit/s±20 ppmOPU2238/237 × 9 953 280kbit/sOPU3238/236 × 39 813 120kbit/sOPU1-XvX × 2 488 320kbit/s±20 ppmOPU2-XvX × 238/237 × 9 953 280kbit/sOPU3-XvX × 238/236 × 39 813 120kbit/sNotes:The OPUk payload speeds are about: 2 488 320.000 kbit/s (OPU1 payload), 9 995 276.962 kbit/s (OPU2 payload) and 40 150 519.322 kbit/s (OPU3 payload).The OPUk-Xv payload speeds are: X × 2 488 320.000 kbit/s (OPU1-Xv payload), X × 9 995 276.962 kbit/s (OPU2-Xv payload) and X × 40 150 519.322 kbit/s (OPU3-Xv payload)
As shown in FIG. 1, the speed of said ODU (Optical Data Unit) is obtained by adding the corresponding overhead bytes, such as cascade supervision byte TCM, management channel GCC byte, and so on, on the basis of the OPU. Therefore, the speeds of ODU should be higher than the corresponding SDH speeds, and the speeds of said ODU are shown as Table 2.
The ODU is the unit to be dispatched, and the packaging of the ODU can guarantee the completeness of its inner service, such as the completeness of the SDH service, however, the traditional SDH dispatch will lose the corresponding section overhead byte and the clock information.
TABLE 2three kinds of ODU and their corresponding speedsODU typeODU nominal bit rateOPU bit rate toleranceOPU1239/238 × 2 488 320 kbit/s±20 ppmOPU2239/237 × 9 953 280 kbit/sOPU3239/236 × 39 813 120 kbit/s Notes:The ODUk bit speeds are about: 2 498 5.126 kbit/s (ODU), 10 037 273.924 kbit/s (ODU2) and 40 319 218.983 kbit/s (ODU3).
The speeds of said OTUs are shown as Table 3, wherein the corresponding management information and Forward Error Correction (FEC) are added on the basis of the ODU.
TABLE 33 kinds of OTU and their corresponding speedsOTU typeODU nominal bit rateOPU bit rate toleranceOTU1255/238 × 2 488 320 kbit/s±20 ppmOTU2255/237 × 9 953 280 kbit/sOTU3255/236 × 39 813 120 kbit/s Notes:The OTUk bit speeds are about: 2 666 057.143 kbit/s (OTU1), 10 709 225.316 kbit/s (OTU2) and 43 018 413.599 kbit/s (OTU3).
The frame period of said OTUs are shown in Table 4, different from the constant frame period of SDH, the frame period of the OTU changes with levels.
TABLE 4the frame period of the OTUOTU/ODU/OPU typePeriodOTU1/ODU1/OPU1/OPU1-Xv48.971 usOTU2/ODU2/OPU2/OPU2-Xv12.191 usOTU3/ODU3/OPU3/OPU3-Xv 3.035 usNotes:The approximation precision of the period reaches to 3 digits after decimal point.
Since OTU/ODU/OPU are different parts in the same frame structure, their frame periods are the same.
For the dispatch of the OTU in the optical synchronous transport system, there are several methods in the industry:
Method 1: according to the specification of OIF (Optical Internetworking Forum) organization, the OTU service can be transmitted in the TFI-5 backplane frame format. TFI-5 defines two speeds: 2.5 Gbit/s and 3.11 Gbit/s, corresponding to STM-16 and STM-20, respectively. For ODU1 with the speed being higher than STM-16, C-4-17C cascaded method is applied to transport and pointer value 522 is regenerated. When asynchronously mapping the ODU service to C-4 container, a mechanism similar to justification needs to be applied: the payload data of the ODU are divided into several data blocks, and each data block is re-assembled to be a structure having justification discrimination bit and justification opportunity bit, and the majority justification using the justification discrimination bit determines whether the justification opportunity bit transports data or not.
Method 2: increase the 2.5 Gbit/s backplane transport speed to 17/16×2.5 Gbit/s, directly apply STM-17 to transport the user signals of ODU1, or apply the method of dividing the STM-68 into several STM-17s to transport the ODU2 signals, or apply the method of dividing the STM-272 into several STM-17s to transport the ODU3 signals.
However, the above two methods have their disadvantages:
(1) Method 1 applies the constant pointer instead of the pointer value to transport, which requires that the position of the header of the ODU frame is also constant in STM-17, thus requiring relatively large buffer memory. Meanwhile, for a speed greater than 2.5 Gbit/s, only the speed of 3.11 Gbit/s can be used to transport, which wastes a lot of bandwidth, has more strict requirements for the components and relatively fewer supportive chips.
(2) Although Method 2 uses the pointer value to avoid large buffer memory, it also changes the general backplane routing speed, which makes the backplane speed a little higher than 2.5 Gbit/s and it can not compatible with the existing systems, moreover, the non-standardized speed lacks the support of well-developed chips, so their application is not flexible.
Therefore, how to uniformly dispatch ODU and SDH service without modifying the framework of the existing devices and without increasing the backplane routing speed is a problem should be solved.